1. Field of the Invention
This invention relates to a high-speed driving method of a liquid crystal for driving a liquid crystal, particularly for gradation display, at a high speed.
2. Related Art
As already known, when two transparent flat plates having transparent electrodes and sandwiching a liquid crystal are placed between two polar zigplates, transmittance of light passing through the polarizing plates changes with the voltage applied across the transparent electrodes.
Since liquid crystal display devices based on the above principle can be shaped flat and are operative with low electric power, they have been widely used in wrist watches, electronic calculating machines, and so forth.
In recent years, they are also used in combination with color filters to make up color display devices in note-type personal computers and small liquid crystal TV sets, for example.
In liquid crystal display devices combined with color filters to display color images, three dots of different colors, namely, red, green and blue, are combined to display desired colors.
A problem with the use of color filters lies in that color filters are very expensive and need a high accuracy when bonded to panels. Moreover, they need a triple number of dots to ensure an equivalent resolution as compared with black-and-white liquid crystal display panels. Therefore, typical liquid crystal color panels require a triple number of drive circuits in the horizontal direction. This means an increase of the cost of drive circuits themselves and the cost for an increased man-hour for connecting drive circuits to the panel at a triple number of points.
Another problem with the use of color filters is their optical transmittance as low as 20% approximately. When color filters are used, the brightness decreases to approximately one fifth, and a large amount of electric power is consumed for back-lighting to compensate the brightness.
There is another problem in conventional liquid crystal devices, namely, slow responses of liquid crystals. In this respect, liquid crystal display devices have been inferior to CRT displays especially when used as TV displays for displaying moving images or as personal computer displays required to quickly follow the movements of a mouse cursor.
Japanese Patent Laid-Open Publication No. hei 1-179914 discloses a color liquid crystal display device to display color images by combining a black-and-white panel and tricolor back-lighting instead of using color filters. This method may enable realization of high-fidelity color images inexpensively, as compared with the method using color filters. In order to realize images without flickers, this method needs an ON-OFF frequency of each color backlight as high as 40 Hz or more, and more preferably 60 Hz or more. As a result, the frequency of liquid crystal display becomes thrice that frequency, namely, 180 Hz, and the cycle of liquid crystal display becomes 5.5 ms. Regarding the ON time of backlight, it must be at least 2 ms, which is two fifths the display cycle, to ensure that a display luminance equivalent to that obtained by using one white fluorescent lamp be obtained by three color tubes, one for each of red, green and blue colors, considering that, when cold cathode ray tubes, such as those using red, green and blue short-afterglow fluorescent materials, optical conversion efficiency relative to input power decreases nearly to a half, but the transmittance increases to five times due to removal of color filters. Therefore, if the duration of 2 ms of the display period 5.5 ms is used for turning on the backlight, then the response speed of the liquid crystal panel is high enough to stabilize liquid crystal display within 3.5 ms. When a graphic image on a display is scanned, for example, even at a high scanning speed as high as 1000 Hz to prevent color striation or defective coloring on the screen, additional 1 ms is further required, and the response speed of the liquid crystal panel must be higher enough to stabilize liquid crystal display within 2.5 ms.
However, in case of liquid crystal display devices using typical nematic TN liquid crystal such as TN liquid crystal or STN liquid crystal, for example, their response speeds are as slow as decades of ms to hundreds of ms.
There are some other proposals to use ferroelectric liquid crystals or anti-ferroelectric liquid crystals to provide liquid crystal panels operative at a high speed. However, no such device has been brought into practice mainly because cell gaps of the liquid crystal must as small as 1 xcexcm or less and are therefore difficult to make. It is known, as means for increasing the response speed of a liquid crystal panel, that the response speed is increased in inverse proportion to the square of the cell gap when the cell gap is narrowed. In case of TN liquid crystals, the response speed can be readily increased to 2 through 3 ms or less by selecting a low-viscosity liquid crystal material and narrowing the cell gap to 2 xcexcm. Even when the cell gap is as narrow as 2 xcexcm, there is an allowance of xc2x10.4 xcexcm for acceptably uniform display, and large-scale panels can be manufactured without serious problems. Therefore, eight-color display with two tones for each color can be realized by using three-color back-lighting and using a narrow-gap TN liquid crystal.
Liquid crystal display devices are more and more widely used as display devices for personal computers in lieu of CRT displays to save the space and electric power. More and more applications for personal computers are designed for multi-color display, and display devices are required to display 16,777,216 colors with 256 tones for each color. Also for use other than personal computers, video images require multi-color display of 64 tones for each color.
There are some schemes for gradation display of liquid crystal panels, such as a method using a voltage to control color gradation or tones in case of liquid crystals like TN liquid crystals or anti-ferroelectric liquid crystals exhibiting relatively moderate changes in transmittance with voltage, a method for display by changing the ratio between the time for display white and the time for displaying black or a method for collecting and averaging a plurality of pixels, in case of liquid crystals such as STN liquid crystals or ferroelectric liquid crystals having only two values of transmittance relative to voltage. However, in the color display method using three-color back-lighting, since the display cycle is relatively short, the method by changing the ratio of white and black display periods of time cannot provide a desired number of tones because the operation frequency of the control circuit increases too high. The method by collecting and averaging a plurality of dots removes the merit of reducing the number of pixels to one third as compared to the color-filter method.
Therefore, in order to realize multi-color display by the color display method using three-color back-lighting, control of tones by voltage control must be used. However, response speeds of liquid crystal panels, in general, are such that changes to or from an intermediate tone are slower by several times or more than changes from white to black or from black to white.
It is therefore an object of the invention to provide a high-speed driving method of a liquid crystal, which increases the response speed sufficiently to enable color display by three-color back-lighting or promise a performance equivalent to or higher than that of CRT displays in reproduction of moving images.
According to the invention, there is provided a high-speed driving method of a liquid crystal for a liquid crystal display device made up of a liquid crystal and two electrodes sandwiching the liquid crystal to display images by applying a voltage based on gradation data between the two electrodes, characterized in applying a predetermined voltage independent from said gradation data across said two electrodes for a predetermined length of time in predetermined intervals.
Especially in color display using three-color back-lighting, it is remarked that the duration of time where all of three color backlight lamps are OFF comes about periodically. By using the fact that, in the duration of time where all of the three color backlight lamps are OFF, the quality of images is not affected by any state of transmittance of the liquid crystal panel, and by applying a voltage to the liquid crystal at a g different from that of a driving circuit of a conventional liquid crystal panel, the response speed of the liquid crystal panel can be increased for all sorts of images including intermediate tone images to realize a bright and low-consumption color liquid crystal panel.